This Variable Inductor was designed and constructed at the National Bureau of Standards in 1911 by Herbert B. Brooks, F.C. Weaver, and Joseph Ludewig. The need for this instrument arose from the Bureau’s work testing current transformers. The inductor has a range of variation of 1 millihenry, can carry 5 amperes continuously, and has low resistance. No instrument then on the market met these requirements. The variable inductor allowed for the variation of the self-inductance of a circuit or the mutual inductance between two circuits, while keeping the resistance constant.

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